Introduction: How to Make a Medieval Trebuchet Out of Cardboard

About: A mechanical engineering graduate-turned software product manager. But there will always be a special place in my heart for making.
Of all the Medieval siege engines, the Trebuchet was by far the most accurate and complex. Trebuchets were capable of throwing 350 pound objects over castle walls. While in this Instructable we won't be making a Trebuchet with that much firing power, the design I will show you is capable of throwing a softball 50-60 feet with a 20 lb counterweight.

Video of me shooting the catapult:
(Sorry the video is sideways, I was having technical difficulties)

Step 1: Materials

The Materials that you will need for this project are as follows:

Cardboard! - You will need A LOT of cardboard for this project unless you are lucky like me and find some quadruple-layer cardboard lying around.

Glue - I used Gorilla Glue and suggest that you use the same.

(Optional) Clamps - If you are using Gorilla Glue, you will most likely need clamps to keep your pieces together while the glue cures.

Pipes/Rods - You are going to need pipes for the axles and joints. I used a 3/4 inch acrylic rod for the axles and 3/4 inch PVC pipe for the axle to slide through.

Washers - You will use these to reinforce the joints that will be holding the counterweight and the the "arm" of the catapult. I used washers with a 7/8 inch hole.

Drill+Drill Bits - Depending on the size of your axle, you will need corresponding Drill Bits to make your holes. I used 13/16 inch and 5/8 inch.

Newspaper - Newspaper works really well when you are making your patterns for the cardboard

Rope/Cord - This will be for the "sling" that throws the projectile

Denim/Canvas Fabric - Also for the "sling"

3/16 Inch Steel Rod - This is also for the sling and only needs to be about 6 inches long

You will also need something to cut your cardboard into shape. I used a scroll saw but a knife or other saw might work.

Step 2: Trebuchet Design

Getting the measurements for your trebuchet is one of the most important steps. Below is a picture of my initial design, with the measurements that I ended up using.

Part A - This is the counterweight and consists of two different pieces, pieces A1 and A2. The more A2 pieces you have, the wider your counterweight is and therefore the more weight you can put in.

Part B - This is the "arm" of the catapult and has two joints, one where it meets the counterweight, and one where it meets Part C

Part C - Raises the throwing mechanism off the ground. Has one joint where it meets Part B. If this part is too short, the counterweight will hit the ground instead of sliding smoothly through the movement

Part D - Provides stability and keeps the trebuchet from knocking itself over. The longer it is, the more stable the trebuchet

Part E - Adds necessary strength to Part C

Step 3: Making Patterns

Here are the exact measurements I used:

Part A1 -
20 in. wide - 20 in. tall

Part A2 -
20 in. wide - 11 in. tall

Part B -
41 in. long
4 in. wide at top / 6 in. wide at bottom

Part C -
41 in. long
6 in. wide at top / 8 in. wide at bottom

*Parts D and E will be covered in another step

Step 4: Cutting Out the Pieces

Trace out/ draw out your shapes and start cutting!

For Part A1, I used 8 layers of corrugation, four on one side and four on the other

For Part A2, I used 16 layers of corrugation

For Part B, I used 12 layers of corrugation

For Part C, I used 24 layers of corrugation, 12 for one side and 12 for the other.

Step 5: Gluing Parts A-C

Apply small amounts of Gorilla Glue and then clamp your pieces together! It takes about 1-2 hours for the glue to cure completely

If you don't have enough clamps, you can lay bricks or other heavy objects on top of the pieces while they dry.

Step 6: Drilling the Holes

There are six holes that need to be drilled in order to complete the throwing mechanism.

Two of the holes need to be drilled on the "arm" of the Trebuchet (Part B). The first hole should be 2-3 inches away from the wider end. I used a 13/16 inch drill bit to make this hole. Using the same drill bit, drill another hole approximately 10 inches from the wider side. It'll take some force, and maybe a couple pounds from a hammer, but shove a piece of 3/4 inch PVC pipe into each of the holes. Pre-cut these pieces of PVC so that they are flush with the cardboard on either side. See first picture for clarity

Drill 5/8 inch holes 2-3 inches from the narrow end of Part C. See second picture for clarity.

Drill 5/8 inch holes into the top of the counterweight and glue the washers around the holes. See third and fourth picture for clarity.

Now cut your 3/4 inch axle into a six inch piece and a 15 inch piece. Take the six inch piece and slide it through one side of the counterweight, then through the "arm" of the trebuchet (in the hole closest to the edge), and through the other side of the counterweight. Take your 15 inch piece and slide it through Part C. Slide the rest of the rod through the other hole in the "arm" and then through the hole on the other half of Part C. See fifth picture for clarity.

Step 7: Parts D+E

Now it is time to attach your supports. For Part D, cut four pieces (four layers apiece) that are 41 inches long. These pieces should be 6 inches wide on one end and 3 inches wide on the other.
Glue these piece to the bottom of Part C as shown on the second picture.

Part E are by far the most important supports. I tried firing the trebuchet without Part E and it had so much power that it ripped off all of the supports I had just put on in Part D.

Cut out pieces that are 4 inches wide and 27 inches long. You will need four pieces that have 12 layers each. Glue these pieces together. Trace the exact angles you will need on both the ends onto each of your pieces. Cut the pieces to shape and glue them in place. See pictures for clarity.

Step 8: Making and Adding the Sling

Now you will be making the sling that holds whatever you are firing.

Depending on how big the items are that you will be throwing, your denim pouch will vary in size. My pouch was about 8 inches by 10 inches.

Take the edge of the side that is eight inches long and fold over about half of an inch. Sow this flap over and do the same on the opposite side. Slide your rope/cord through these folds. both these strings should be the same length. Take one of the strings (after you have fed it through) and tie it so that it makes one big loop. Leave the other string untied. See picture for clarity.

Drill a hole in the "arm" of the catapult on the side opposite of the counterweight. This hole should be just big enough for you rope/cord to fit through.

Take the untied rope/cord and feed it through this hole. Once it is run all the way through the hole, tie it to the other end of the rope/cord.

Take your six inch rod and poke it into the cardboard near the hole you just drilled.

Step 9: Firing and Adjusting the Trebuchet

I decided to make a video for this step because otherwise it would be too confusing to describe.

Firing the Trebuchet

Adjusting the Trebuchet

For more power, add more weight to the counterweight. Here are some ideas for filling your counterweight:

- Bricks work well, but only if they fit width-wise
- thick gravel will work sometimes, but sand will seep through the corrugation

I ,however, used a 20 pound shirt of maille armor that I made last year. It fits perfectly and does not shift during the firing sequence.
Gorilla Glue Cardboard Contest

First Prize in the
Gorilla Glue Cardboard Contest